Make sure you get the minimum drivers required by the new system, so it can connect to the internet. This means network drivers for sure, and storage drivers if the disk is SATA.
If you bought a Dell, you can plug in your Service Tag number on the Dell site, and go to a page that lets you download every required driver for your particular machine. I'm doing this right now with a brand new Dell 6400 laptop. I save a disk image of the factory system, then wiped the C: partition clean for a reinstall without all the bundled junk. The performance increase is quite substantial.
You must have the WinXP registration code available. If you had it installed on different hardware, the install will force you call Microsoft and grovel for a reset of your token. XP hashes the hardware and uses it as part of the registration key. When the hardware changes, the install won't register until you grovel.
If you have a bootleg copy of XP or a cracked serial number, give up now. MS is wise to that shit, and the Windows Update won't work.